Commitments and Contingencies, Environmental Contingencies (Details) (USD $)			1 Months Ended	9 Months Ended		1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury and Air Toxics Standards (MATS) Rule [Member] MW	Sep. 30, 2012 Ashland MGP Site [Member]	Dec. 31, 2011 Ashland MGP Site [Member]	Oct. 31, 2012 Ashland MGP Site - Phase I Project Area [Member] acre	Sep. 30, 2012 Ashland MGP Site - Phase I Project Area [Member]	Sep. 30, 2012 Ashland MGP Site - Sediments [Member]	Sep. 30, 2012 Other MGP Sites [Member]	Dec. 31, 2011 Other MGP Sites [Member]
Manufactured Gas Plant (MGP) Sites [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own				2
Liability for estimated cost of remediating sites				$ 104,300,000	$ 104,300,000		$ 40,000,000		$ 3,600,000	$ 3,300,000
Number of acres of land to be conveyed to the State of Wisconsin and tribes (in acres)						1,390
Estimated cost of remediating site, low end of range								63,300,000
Estimated cost of remediating site, high end of range								77,100,000
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)								50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)								30.00%
Liability for estimated cost of remediating sites, current	$ 31,650,000	$ 30,699,000		$ 26,600,000	$ 26,600,000
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)				4Y
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)				6Y
Number of identified MGP sites under current investigation and/or remediation									3
Environmental Requirements [Abstract]
Generating capacity (in MW)			25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)			3Y
Number of years before affected facilities must demonstrate compliance, high end of range (in years)			4Y